Biogas Production from Small Farm Cow Manure by High Solid Three-Stage Anaerobic Digester

Abstract

High solid anaerobic digestion (HS-AD), a so-called semi-dry or dry anaerobic system is an alternatively potential option for substrate degradation under less water content. The solid-state anaerobic process is categorized into the semi-solid or high solid-state for having total solids between 10% and 20% in a system and solid-state for a system having total solids higher than 20%. This thesis aimed to study the three-stage high-solid anaerobic digestion (TSHS-AD) at ambient temperature with cow manure. Biomethane Potential was conducted at the various substrate to inoculum (S: I) ratios of 2:1, 4:1, and 6:1 on a volatile solid (VS) basis, each of which was conducted at different total solid concentrations (15%, 20%, and 30%). The highest cumulative methane of 343.97 mL-CH4·gVS-1 was achieved at 2:1-S: I ratio and 15 %TS. The first-order hydrolysis constant (kh) decreased, when the initial %TS and S: I ratio was increased. Consequently, methane production was correspondingly reduced. According to the modified Gompertz parameters, the maximum biogas production rate (Rmax), the time period for 90% of total biogas yield (T90), and the effective biogas production period (Tef) were increased, when the initial %TS and S: I ratio was decreased. Additionally, it was found that the lag phase () became shorter by using less initial % TS and S: I ratio. Meanwhile, TSHS-AD of three tanks in a row operated at S: I 2:1 and 15 %TS (20g-TS·L-1 reactor1(R1)) provide a maximum methane yield of 316.09 mL-CH4·gVS-1. The methane yield was increased by stepwise increasing initial manure concentrations from 5 g-TS·L-1-R1 up to 20 g-TS·L-1-R1. However, continuously increasing initial manure concentration to 25 g-TS·L-1-R1 and 30 g-TS·L-1-R1 could cause a decrease in methane production rate. Methane production from compact TSHS-AD of cow manure produces 309 mL-CH4.gVS-1, which is rather similar to methane yield obtained from 1-L TSHS-AD. The stage separation of TSHS-AD reduces inhibitors in the methanogenesis step. Moreover, it could create a full microbial function in each stage. Accumulation of VFA and semi-continuous feeding have considerable influence in enhancing anaerobic microbes involved in the biogas system. The annual income from biogas has the greatest impact on the investment attractiveness of TSHS-AD, according to the computation of the internal rate of return (IRR), Net present value (NPV), and the payback time (PB). The results obtained in this research could be potentially helpful to scale up the TSHS-AD system